Democrats are taking aim at Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE’s perceived ties to Russia.

On the campaign trail, Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE’s camp is painting the Republican nominee as the “pro-Kremlin” candidate.

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Clinton on Monday said the investigation into Russian involvement in the hack of the Democratic National Committee (DNC) “raises further questions about Trump,” who “very early on allied himself with” Russian President ­Vladimir Putin’s policies.

Her running mate, Sen. Tim Kaine Timothy (Tim) Michael KaineTrump meets with potential Supreme Court pick Amy Coney Barrett at White House Names to watch as Trump picks Ginsburg replacement on Supreme Court Barrett seen as a front-runner for Trump Supreme Court pick MORE (D-Va.), last week challenged Trump on his foreign business ties and accused him of “[encouraging] Russia to get in here and screw around with our elections.”

Meanwhile, on Capitol Hill, lawmakers are urging the FBI to open investigations into Trump’s role in the attack on the DNC.

Senate Minority Leader Harry Reid Harry Mason ReidOn The Trail: Battle over Ginsburg replacement threatens to break Senate Democrats fear Russia interference could spoil bid to retake Senate Graham signals support for confirming a Supreme Court nominee this year MORE (D-Nev.) cited “evidence of a direct connection between the Russian government and Donald Trump’s presidential campaign.”

Trump has flatly and repeatedly denied any ties to Russia.

“I have nothing to do with Russia. I don’t have any jobs in Russia. I’m all over the world but we’re not involved in Russia,” he said during a late July interview with CBS Miami.

The attacks from Democrats are escalating just as the presidential campaign enters the final sprint to Election Day.

The accusations that Trump has a cozy relationship with Russia are a real vulnerability for his campaign, polls suggest.

According to a YouGov poll conducted after the DNC hack, 40 percent of Americans think Trump is “too friendly” toward Russia — although those results were starkly divided by party. Democrats, at 62 percent, were much more likely than Republicans, at 14 percent, to view him as too friendly with the Kremlin.

Strategists say Russia is an effective line of attack for Democrats because it flips the script on Trump’s usual attacks against Clinton: that she’s untrustworthy, that she’s corrupt and that the election will be rigged in her favor.

“It’s good politics because it takes the pressure off Hillary and puts it on Trump,” said Democratic strategist Brad Bannon. “What’s good for the goose is good for the gander.”

“This is just playing the cards that Trump has given them. This is turning his argument [that the election will be rigged] against him,” said Jim Manley, a Democratic strategist who backs Clinton.

Questions about Trump’s ties to Russia have lingered throughout the campaign, thanks to a series of remarks in which the businessman appeared to praise Putin. Trump in his campaign speeches often talks about how it would be “great” for the United States to get along with Russia.

Speculation about Trump’s ties to Russia exploded after the hacked DNC emails were released on the eve of the Democratic National Convention in Philadelphia this summer. The leak embarrassed party officials and led to the resignation of Rep. Debbie Wasserman Schultz (D-Fla.) as chairwoman.

After the leak, Trump said that he “hoped” Russian hackers obtained the tens of thousands of emails Clinton deleted from her private email server during her time as secretary of State.

Trump later said he was joking, but appeared to pay a price for the remark: Thirty-one percent of Americans said they were more concerned that Trump would suggest Russian hackers should try to get the emails than they were that Russia was able to steal the information in the first place.

Although the Obama administration hasn’t officially attributed the DNC hack to Russia, security experts — and, speaking anonymously, administration officials — say they are virtually certain that Kremlin-backed intelligence groups were involved.

How the pages got into the hands of the anti-secrecy platform WikiLeaks, which published them in July, is muddier — but the timing of the dump led many to suggest that it was an effort to help Trump.

Some argue it’s likely Russian spies intentionally provided the DNC pages to WikiLeaks to ensure a Trump White House.

“I often quote a great saying that I learned from living in Arkansas for many years: ‘If you find a turtle on a fence post, it didn’t get there by itself,’ ” Clinton said Monday. “I think it’s quite intriguing that this activity has happened around the time Trump became the nominee.”

Now Democrats are suggesting that either Trump or his surrogates may have had a direct role in the theft and release of the pages.

In addition to the Reid letter, the ranking members of four House committees on Wednesday asked the FBI to investigate whether “connections between Trump campaign officials and Russian interests may have contributed to these attacks in order to interfere with the U.S. presidential election.”

“Serious questions have been raised about overt and covert actions by Trump campaign officials on behalf of Russian interests,” they wrote. “It is critical for the American public to know whether those actions may have directly caused or indirectly motivated attacks against Democratic institutions and our fundamental election process.”

Former Trump campaign chairman Paul Manafort has drawn the most scrutiny.

Manafort was forced to resign last month amid reports that he helped a pro-Putin political party in Ukraine secretly funnel more than $2 million to two lobbying organizations in Washington.

Also last month, Trump confidant Roger Stone appeared to confirm suspicions that he is in contact with WikiLeaks’s controversial founder, Julian Assange.

Assange has warned that he is sitting on another significant document dump that will damage Clinton.

Asked about the contents of those documents, Stone replied, “I actually have communicated with Assange. I believe the next tranche of his documents pertain to the Clinton Foundation but there’s no telling what the October surprise may be.”

Assange, who has made no secret of his distaste for Clinton, has been accused of being an ally of the Russian government. He once hosted a television program on a Russian propaganda network and last week was the subject of a long New York Times profile suggesting many of his data dumps directly benefit Russian interests at the expense of the West.

Critics of Trump have also pointed to reports that his daughter, Ivanka Trump, recently vacationed with Rupert Murdoch’s ex-wife, Wendi Deng Murdoch, who is rumored to be dating Putin.