Trump's tweets came a day after Ryan effectively disavowed him without formally pulling his endorsement -- and a poll showed Trump trailing Clinton by double digits ahead of November's election.
A 2005 video that surfaced Friday of Trump bragging about groping women prompted a wave of Republican lawmakers to withdraw their support in a last-ditch effort to save their control of Congress. Others, including Republican National Committee Chairman Reince Priebus and vice-presidential nominee Mike Pence, are standing by Trump.
Trump tweeted Tuesday that a lack of support from Ryan has made it "hard to do well," in an apparent acknowledgment of the political damage of increasing GOP infighting.
"With the exception of cheating Bernie out of the nom the Dems have always proven to be far more loyal to each other than the Republicans!" Trump said in another follow-up tweet.
While the Republican Party is fracturing, Clinton was pulling in prominent Democratic figures and officeholders to campaign for her in battleground states in a show of unity and in an attempt to close off any path to victory for Trump.
She's appearing in Miami Tuesday with former Vice President Al Gore. Her husband, former President Bill Clinton was dispatched to Iowa this week and Vice President Joe Biden was hitting Nevada.
President Barack Obama is appearing Tuesday in Greensboro, North Carolina and heading to Ohio on Friday to raise money for the state's Democratic Party organization. First lady Michelle Obama will be in New Hampshire on Thursday.
Many House Republicans, meanwhile, are getting worried that Trump is damaging their party's prospects to retain control of the Senate and to keep their sizable majority in the House.
"It will be very difficult for Trump to win," said Rep. Charlie Dent of Pennsylvania, a leader of House Republican centrists. He told his colleagues on the Monday conference call that it was time to distance the party from Trump.
In the long run, said Dent, "there is a battle going on for the soul of the party."
"There will be a reckoning after the election," he added.
Some House Republicans appear torn over how to proceed.
Rep. Dennis Ross of Florida, a senior deputy majority whip, says he understands that Ryan is doing this because the speaker believes it is the best way to protect the House majority.
"But we need to defeat Hillary," said Ross, citing the future direction of the Supreme Court and other important issues tied to the White House race.
Even so, Priebus and other RNC members made clear that they're still backing Trump.
"Our support of the nominee is unbroken, and that's what was communicated by the chairman," said Bruce Ash, an RNC member from Arizona. "There's not a member I talked to that feels any differently."
Ash said that Ryan made a mistake by giving up on Trump and that the speaker reacted "with his stomach and heart, rather than with his mind."
By contrast, Ash said Senate Majority Leader Mitch McConnell has approached this "very politically astutely," because he "put his head down, waited for the bullets to fly and the circus to pass by."
McConnell on Monday simply declined to address the presidential race at all during a public appearance in Kentucky.
Either way, the split leaves Senate Republicans running for re-election in a very complicated spot.
Vulnerable candidates have split in their approach to Trump, with nearly a third of the chamber's Republicans now saying they will not vote for Trump and some urging him to drop out of the race.
Even those who remained in Trump's corner, like Richard Burr of North Carolina, made sure not to repeat New Hampshire Senator Kelly Ayotte's gaffe last week that Trump was "absolutely" a role model for children.
"Both candidates proved they are not role models for the next generation, but we don't get to pick now any additional folks to run, so we have to pick who we best think meets the way forward in the future," Burr told reporters in North Carolina Monday.
Roy Blunt of Missouri remained one of the most pro-Trump of the vulnerable senators, quickly praising him for apologizing for his remarks on the 11-year-old tape, even as a number of his colleagues -- Ayotte, Rob Portman of Ohio, John McCain of Arizona and others took the opportunity to leap off the Trump train.
Sen. Pat Toomey of Pennsylvania, who is being challenged by Democrat Katie McGinty, faulted both Trump and Clinton and hasn't said which candidate he'll support.
Whether candidates will be able to distance themselves from Trump and still prevail is unclear.
"As Trump's numbers crater many House and Senate Republicans will find themselves on the wrong side of polling trends," said Joshua Huder, a senior fellow at the Government Affairs Institute at Georgetown University in Washington. "They can only distance themselves so much."
"Either it divides their base or it deflates turnout. Neither bodes well for Republican incumbents sitting on or near the fence," Huder said.
Trump, who has campaigned as an outsider intent on shaking up Washington, has successfully channeled discontent among Republican voters and insisted that the Republican establishment isn't that relevant to his White House prospects. "I can win one way or the other," Trump said in an NBC News interview in June.
Trump has heralded unscientific online reader polls on news websites that showed him winning Sunday's second presidential debate. In fact, three more methodologically sound surveys by professional pollsters showed Clinton was seen as the better debater by margins ranging from 5 to 14 percentage points.
Nationally, Clinton has about a 5-point edge on Trump in a race that includes third-party candidates, according to the RealClearPolitics poll average. The forecaster FiveThirtyEight on Monday gave Clinton an 82.9 percent change of winning in its polls-only model, approaching her high of 89.2 percent in mid-August. FiveThirtyEight gave Democrats a 52.6 percent change of winning back the U.S. Senate.
