"I'd think long and hard about getting in that truck with Martha's opponent," the president said. "It might not take you where you want to go."
Obama tied Brown's agenda to "Washington Republicans" who are against financial reform, clean energy and health care reform.
"When the chips are down, when the tough votes come, on all the fights that matter to middle class folks of the Commonwealth, who is going to be on your side?" he asked.
Coakley today addressed the issue of economy that Brown has made the defining feature of his populist campaign.
"I wish there were easy answers to the tough problems we have," she said at a breakfast today commemorating Martin Luther King Jr. "Do not forget that they are problems that were not created by, but inherited by, our president, Barack Obama."
Candidates on both sides of the tight race have seen money and resources pouring in for their campaigns. Brown raised more than a million dollars online every day last week, according to a Republican source familiar with his fundraising. Bush administration official Karl Rove told his twitter followers to phone bank. Sen. John McCain called on to his supporters to back Brown.
On the other side, Democratic groups placed more than 500,000 calls on Saturday. Former president Bill Clinton appeared with Coakley at two campaign events, and Kennedy's widow, Vicki, appeared in a television ad to support the attorney general. Last week, Obama taped a robo-call in support of Coakley.
"In Washington, I'm fighting to curb the abuses of a health insurance industry that routinely denies care. I'm fighting for financial reforms to stop Wall Street from playing havoc with our economy. I'm fighting to create a new clean energy economy," the president said in the call.
"And it's clear now that the outcome of these and other fights will probably rest on one vote in the United States Senate," he said.
National Democrats are shocked that the race seems this close, given that Democrats outnumber Republicans in Massachusetts 3-1. Some say it is less about the president and more about a weak candidate.
But some Republicans say the mere closeness of the race spells troubles for Democrats.
"Whether Coakley wins or loses, the message is the same, health care. This health care package is death for candidates," said conservative commentator Tucker Carlson.
As Massachusetts voters head to the polls Tuesday, both candidates are stepping up their campaigns.
Coakley today invoked the memory of civil rights leader Martin Luther King Jr., to make her pitch.
"If Dr. King were here today, he'd be standing with us," she said. "And I know that he would be standing with us on the front line for health care, not as a privilege, but as a right."
ABC News' Jake Tapper, Jonathan Karl and Kristina Wong contributed to this report.