The conservative Club for Growth is putting out a new ad looking to boost Sen. Mike Lee, R-Utah, as he faces a tougher-than-anticipated challenge from independent Evan McMullin.

The video, backed by an $800,000 reservation, will begin airing statewide Wednesday through Election Day. The 30-second clip features former Gov. Jon Huntsman, who served one term and was later the U.S. ambassador to Russia during the Trump administration.

"There's a lot of noise out there right now, so let me tell you what I know about Mike Lee. I've worked with Mike since I was governor, and I know his heart. He has the unshakeable integrity that's all too scarce in politics. You might not always agree with Mike. I don't. But we can all agree that principled leadership and fidelity to the Constitution are paramount in difficult times. Let's send Mike Lee back to the Senate," Huntsman says in the ad.

The late buy suggests 11th-hour handwringing by some of Lee's allies over McMullin's bid.

Lee recently went on Fox News to plead with Sen. Mitt Romney, R-Utah, for his endorsement, and the FiveThirtyEight polling average only has Lee up by about 8 points after he won reelection in 2016 by more than 40.

McMullin has centered his challenge around Lee's communications discussing methods to challenge the 2020 election results after the race was over, accusing Lee of shedding his label as a "constitutional conservative." Lee notes he did not ultimately reject to certifying the Electoral College results.

He and his allies have accused McMullin -- who previously worked as a Republican congressional aide and then ran for president in 2016 as an independent -- of now being a closet Democrat, noting that the state Democratic Party endorsed him and did not run a candidate of its own. McMullin, though, has said he wouldn't caucus with either party in the Senate.

-- ABC News' Tal Axelrod

The newest John Fetterman ads feature words of support from former President Barack Obama, the latest high-profile surrogate to involve himself in Pennsylvania's crucial Senate race.

"When the fate of our democracy and a women's right to choose are on the line, I know John will fight for Pennsylvanians," he says in both ads.

Notably, in the radio ad, Obama touts Fetterman's service on the state Board of Pardons, which opponent Dr. Mehmet Oz attacks repeatedly by accusing Fetterman of endangering Pennsylvanians by voting to commute the sentences of some convicted murderers.

"As Lieutenant Governor, John chairs the Board of Pardons and has helped people who were wrongfully convicted," Obama says.

In a statement, Fetterman said, "I am deeply grateful for President Obama's support in this race. I was proud to be one of the only elected officials in western Pennsylvania to endorse President Obama in the 2008 presidential primary, and it is truly an honor for him to get behind our campaign and cut ads for us across the commonwealth."

While the campaign noted in a press release that the ads will appear "across Pennsylvania" in the coming weeks, the radio hits began airing in the Philadelphia market this week.

–ABC News' Will McDuffie

Wednesday's placements are the second round of ad spending by the new PAC, which was launched in late September and kicked off its first round of placements in the same Senate races earlier this month.

According to ad tracking firm Ad Impact, Make America Great Again Inc.'s biggest investment in the latest ad placement was a $1 million ad buy in the race between Democrat Tim Ryan and Trump-endorsed J.D. Vance in the Ohio Senate race, followed by $770,000 in spending on the rivalry between Democrat John Fetterman and Trump-endorsed Dr. Mehmet Oz in the Pennsylvania Senate race.

Read more here.

–ABC News’ Soo Rin Kim

There's the campaign Republicans are running on inflation, crime and the Biden agenda -- a sprawling and broad case against Democratic governance that's being adjusted to fit particular circumstances in races across the map.

Then there's the campaign Democrats are running -- on abortion rights and against far-right Republicans, but also at times against the Biden White House on the southern border and the handling of inflation and the urgency under which all that and more is being addressed.

Enter, again, President Joe Biden. He makes a rare-for-this-cycle trip to battleground Pennsylvania on Thursday to campaign for Democratic Senate candidate John Fetterman-- and also to provide messaging cover for the many Democrats he isn't campaigning for in person, either because of their choice or the constraints of time.

The streamlined messaging includes stronger commitments on abortion, including Biden's vow this week that, in January, he will sign a bill to codify Roe v. Wade -- contingent on holding the House and picking up at least two seats in the Senate.

–Read more here.

–ABC News’ Rick Klein