Republican U.S. Rep. Erik Paulsen has been trying hard for months to play up differences with and keep distance from President Trump during a tough re-election race, so it was a surprise when Trump endorsed the congressman's re-election effort.

The twist, which came during a late-night presidential tweet Monday, drew a more ecstatic response Tuesday from the campaign of Democratic challenger Dean Phillips than it did from Paulsen himself.

"Trump is Trump. He's going to do what he's going to do," Paulsen said in a telephone interview in between campaign appearances at senior centers in the 3rd Congressional District.

"President Trump does not endorse moderates," said Phillips, a businessman and first-time candidate giving Paulsen his toughest test in a decade.

At least $9 million has been spent by outside groups as of last week on top of the $7 million in documented candidate spending through mid-October. Much of the outside spending has been done on Paulsen's behalf in searing ads against Phillips.

Rep. Erik Paulsen responds to a question during a debate with DFL congressional candidate Dean Phillips at the DoubleTree Park Place in Minneapolis on Aug. 21, 2018. Lacey Young | MPR News file

But Democratic allies are coming in late to help Phillips in the quest to flip the seat. The Democratic Congressional Campaign Committee is airing its first ads calling Paulsen a special-interest sellout.

The race is one of four top-tier U.S. House contests this year in Minnesota. The outcomes could be pivotal in determining which party controls Congress.

Fellow GOP Rep. Jason Lewis has fully embraced Trump in his bid for a new term representing the 2nd Congressional District, a race that has him in a rematch with Democrat Angie Craig.

Two Republicans vying for open seats — Pete Stauber in northern Minnesota and Jim Hagedorn in southern Minnesota — also tout their backing of Trump in their campaigns to represent districts where the president fared well two years ago. Their respective opponents are DFLers Joe Radinovich and Dan Feehan.

DFL candidates Angie Craig and Dean Phillips stand on stage at a rally for Vote Common Good at Veterans Park in Richfield on Oct. 10, 2018. Tony Saunders | MPR News

Trump campaigned in the 8th District for Stauber and the 1st District for Hagedorn, drawing thousands to rallies. But Paulsen skipped both visits, and he wasn't mentioned from the stage like the other GOP candidates in Minnesota's competitive races.

In fact, Paulsen doesn't hide the fact that he wrote in a candidate — Florida Sen. Marco Rubio — rather than vote for Trump in 2016.

Paulsen said he learned of the Trump endorsement when it appeared on Twitter. He insists he didn't ask for the nod and wasn't told it was coming.

It cuts both ways: Trump is working to rev up his base on behalf of endangered incumbents such as Paulsen as the midterm election draws closer. But Paulsen is being tested in a district Trump lost by almost 10 points and where some Republicans and independents remain wary of the president.

"I wish the president would endorse my positions on trade and protecting Minnesota's Boundary Waters," he said. "These are two issues that are critically important to Minnesota."

Paulsen declined to say if he would attend a late-campaign rally with Trump if one comes together, saying he'd focus on "connecting with my voters."

"I'm going to continue to focus on my own race and work hard all the way through the finish line," he said.

Phillips said the Trump seal of approval makes it harder for Paulsen to keep his distance.

"The quotes have not matched the votes. I think President Trump's endorsement of Congressman Paulsen is to be expected because Paulsen has been a loyal soldier," Phillips said.

Meanwhile, Paulsen and Republican groups have worked to link Phillips to embattled Rep. Keith Ellison, a congressman and the Democratic nominee for attorney general. Ellison trails in his race and has struggled to overcome an ex-girlfriend's allegations of abuse, which he denies.

"Dean Phillips should be honest with voters about whether he'll vote for Keith Ellison or not," Paulsen said.

Phillips isn't making his vote in that race known.

"The only person that I know I'm voting for on Nov. 6 right now is the guy you're speaking with, Dean Phillips," he said.

Phillips said the comparison is misplaced.

"Congressman Ellison and my race, we have nothing to do with one another. He's running a statewide race for attorney general," Phillips said. "Four million Minnesotans will be the jury, if you will, on that. I'm running my race in the 3rd District about the issues important to the people I'm hoping to serve. And I'm running against Erik Paulsen."