Questions about Russian influence over the presidential campaign have united Democrats in pursuit of President Trump, while Republicans are defending him warily.

The president escalated the partisan war of words this weekend when he alleged that his predecessor, former President Obama, may have wiretapped Trump Tower during the campaign. Republican fingers have increasingly pointed at Obama administration holdovers as the source of damaging anti-Trump leaks.

House Intelligence Committee Chairman Devin Nunes, R-Calif., announced Sunday that looking into these allegations will be part of the ongoing congressional investigation. Nunes has previously warned against allowing the Russia probe to become a "witch hunt" or "almost like McCarthyism."

Former National Intelligence Director James Clapper denied seeing evidence of either anti-Trump wiretapping or collusion between the Russians and Trump's campaign.

But the past week underscored how fragile Republican unity on Trump and Russia is. When Attorney General Jeff Sessions' two meetings with the Russian ambassador were revealed, Republicans defended him from Democratic charges of perjury and demands that he resign — but in many cases joined calls that he recuse himself from the Russia investigation.

House Oversight Chairman Jason Chaffetz, R-Utah, said Sessions "should clarify his testimony and recuse himself." Sen. Rob Portman, R-Ohio, said such a recusal would be "best ... for the country." Rep. Mike Coffman, R-Colo., said Sessions was guilty of a "grave omission by not disclosing his meeting with the Russian ambassador last year."

There were significant exceptions, of course. House Speaker Paul Ryan, R-Wis., said Sessions need only recuse himself if he becomes a subject of the investigation. And Republicans defended Sessions' character.

The attorney general ultimately announced he would recuse himself.

"Democrats wouldn't have piled on Eric Holder or Loretta Lynch like that," complained a Republican operative. Sessions is popular with Republicans on Capitol Hill and his answers to Russia questions during his Senate testimony could plausibly be read to be consistent with meeting the ambassador outside his role as a Trump campaign surrogate.

Even Sen. Al Franken, D-Minn., the liberal lawmaker whose Russia question is at the center of the controversy, has stopped short of saying Sessions lied.

"I would like to give him some benefit of the doubt," Franken said of the attorney general.

Nevertheless, Sen. Lindsey Graham, R-S.C., said that if there was anything to the Russia claims his "dear friend" Sessions "cannot make this decision about Trump."

If this is the case with Sessions, what if there was a Russia development that did not involve an administration official with the level of Republican support Sessions enjoys?

Depending on where the facts lead, Russia has the potential to drive a wedge between Trump and congressional Republicans for several reasons.

Many GOP lawmakers disagree with a rapprochement with Vladimir Putin's Russia. They would like to see the administration take a harder line than Obama did, but they are worried that Trump will not pursue such a policy.

Graham, Sen. John McCain, R-Ariz., and Sen. Marco Rubio, R-Fla., have all criticized Trump on Russia and all expressed reservations about Secretary of State Rex Tillerson's business ties to the country, despite ultimately voting for his confirmation.

Republicans were similarly split on former National Security Adviser Mike Flynn, the first senior Trump administration official to lose his job over contacts with Russia.

There is also anxiety over what the investigations will actually uncover. "We're all waiting for the other shoe to drop," the operative said. Trump's campaign was loosely organized. The president took weeks to publicly accept intelligence community's conclusions about Russian hacking during the campaign. Some Republicans are uncomfortable with Trump's rhetoric about Russia.

Other Republicans complain that a failure to circle the wagons is in the party's DNA. GOP strategist Ford O'Connell noted that this has been evident even during lesser controversies, like Mitt Romney's reference to "binders full of women."

"They're not doing the party or themselves any favors," he said.

All of these factors make at least some Republican members of Congress less gung-ho in their defenses of Trump than they otherwise could be, although they are still broadly supportive of the president.

Trump appears to be gearing up for a major showdown with his political opponents. It remains to be seen whether he can keep his party behind him for the duration of the battle.