Mitt Romney's selection of Paul Ryan as his running mate for the presumptive Republican presidential ticket brought an immediate response from President Obama's re-election campaign, but it could also have a trickle-down effect on several races in the battle for control of the Senate.

Ryan, a seven-term congressman from Wisconsin, has made waves in recent years with controversial budget proposals and his plan to overhaul Medicare.

Democrats have attacked Ryan for several years, especially since he became chairman of the House Budget Committee in 2010 after Republicans won a majority in the House, but now that Ryan is on the likely GOP presidential ticket, the attacks take on a whole new meaning.

Besides the ramifications of Romney's decision on his own race, there's also the question of the effect on the Senate races, where Republicans are hoping for a net gain of four seats in order to take back the majority, while also taking the White House and maintaining control of the House.

ABC News has identified six key toss-up Senate races this year: Massachusetts, Nevada, Virginia, Missouri, Montana and Ryan's home state of Wisconsin. While it's unclear how Ryan's presence on the ticket will play out in any of these races, it's becoming clear that Democrats will be using this latest development as an attack line going forward.

Virginia

Polling has found the Virginia senate race to be neck and neck, and with the presidential race very tight there as well, both candidates have approached their parties' nominees with a sense of caution - they're open in their support, but it's not always highlighted, and it's not always unwavering. Democratic Senate candidate Tim Kaine has highlighted his difference of opinion with Obama on off-shore drilling, for example.

Like many other Democrats, it appears as though the Kaine campaign sees an opportunity to hurt their opponent, former Virginia Sen. George Allen, by tying him to Ryan's plan. Allen was present at the Romney-Ryan announcement, and Kaine quickly released a statement hitting him for having a fiscal approach that would "gut Medicare resources for millions of American seniors."

"By standing with Paul Ryan today, George Allen continues to embrace a plan that would force hundreds of thousands of Virginia seniors to pay nearly $6,000 more each year in health care. Budgets are about priorities and George Allen's approach would gut Medicare resources for millions of American seniors, devastate investments for education and infrastructure that grow our economy, while defending irresponsible tax breaks for the wealthiest that ballooned our deficit and drove up our debt," Kaine for Virginia spokeswoman Brandi Hoffine said in a statement.

Missouri

Missouri is considered to be a safe bet for Republicans in this presidential cycle - Obama is unpopular and polling has consistently found Romney with a strong lead. Recent polls have shown incumbent Democratic Sen. Claire McCaskill in trouble (although Democrats have been buoyed by victory of Rep. Todd Akin in the Republican primary last week, as conventional wisdom suggested he was the easiest candidate for McCaskill to beat) and she's been the target of attacks from outside groups for a long time.

McCaskill appears to view the Ryan announcement as an opportunity for a new attack line on Akin. Today she tweeted "the part of Ryan-Akin budget I hate the most? Cutting Medicare and then giving those cuts to the mega wealthy. Wrong." Expect McCaskill, who is frequently described by her colleagues as a fighter, to hit hard with this new messaging.

Massachusetts

Republican incumbent Scott Brown has stayed away from Mitt Romney thus far; as a Republican running in a Democratic heavy state, Brown will need at least some Obama voters to cross over and vote for him. Brown voted no on the Ryan budget in Congress, and he even went so far as to pen an op-ed in Politico explaining his reasoning, so he should in theory be able to withstand any attacks from Elizabeth Warren, his Democratic opponent, tying him to the controversial proposal.

"While I applaud Ryan for getting the conversation started, I cannot support his specific plan - and therefore will vote 'no' on his budget," Brown wrote in an op-ed in Politico in May 2011.

"Why can't I go along with the Ryan Medicare plan? First, I fear that as health inflation rises, the cost of private plans will outgrow the government premium support - and the elderly will be forced to pay ever higher deductibles and co-pays," he wrote. "Protecting those who have been counting on the current system their entire adult lives should be the key principle of reform."

For now, it appears as though Warren is not attempting to tie her opponent directly to the budget, but reiterate her ties to Obama.

"The choice is clear," Warren said in a statement. "Mitt Romney and Paul Ryan will work to make the rich and powerful, richer and more powerful. I'm standing with President Obama to work for our families, to invest in our kids, and to give our small businesses a fighting chance to succeed because I believe that's how we build a strong foundation for our future."

Nevada

Nevada's economic woes are well documented, and the Romney campaign hopes that said woes will put the state in the Romney column, and the same hope exists down ballot in the tight senate race between incumbent Sen. Dean Heller and Democratic Rep. Shelley Berkley. Heller voted for the Ryan budget twice - once in the House, once in the Senate - and Berkley's campaign has already been attacking him for it.

For Berkley, who is under formal investigation from the House Ethics Committee after being accused of using her office to help her husband's medical practice (she saved a Las Vegas area kidney transplant center, a move that appears to have benefited her husband who is a kidney specialist), the Ryan announcement could be a welcome opportunity to shift the focus. Expect Berkley to continue to charge her opponent with supporting a plan that "would end Medicare as we know it."

Montana

Democrats have already begun to highlight an ad released by Montana Republican Senate candidate Denny Rehberg earlier this year in which the candidate specifically called out Ryan's budget as potentially harmful to seniors.

"Rehberg refused to support a Republican budget plan that could harm the Medicare programs so many of Montana's seniors rely on," the ad, titled "Montana First," said.

Shortly after Romney's announcement, the Democratic Senatorial Campaign Committee (DSCC) sent out a press release highlighting that ad, and there's no reason to believe they won't continue to play up the comments.

Wisconsin

This is of course the state where Ryan's presence on the ticket is most likely to boost Republican's chances of winning the senate seat. The Republican candidate is not yet known in this race- the primary will take place Tuesday and a challenger for Democratic Rep. Tammy Baldwin will emerge from a crowded field. Republicans hope Ryan's presence on the ticket will boost turnout for Romney in the presidential race and turn the state red for the first time in a presidential election since 1984.

As it's likely he'll be campaigning in Wisconsin a lot, Ryan can be expected to hit the stump at least a couple of times for the chosen Republican Senate candidate, and his presence in the race could be the boost Republicans are hoping for across the board.

It's important to note that the population of residents 65 and over in these states is within a couple percentage points of the national average of 13 percent in each instance, so there is not an obvious state where just in terms of numbers, Ryan's presence could be a concern.