By Susan Cornwell

WASHINGTON (Reuters) - Barbara Comstock is trying to untether herself from Donald Trump in her re-election bid for the U.S. Congress, but the Virginia Republican's struggles show how difficult that can be.

Comstock represents a wealthy House of Representatives district in northern Virginia where Trump has become a burden, one that her opponent is wrapping around Comstock's neck.

In local campaign ads, Democratic challenger LuAnn Bennett takes every opportunity to tie Comstock to the New York real estate developer and Republican presidential nominee.

The strategy may be working. The Cook Political Report, a non-partisan election tipsheet, moved the Comstock-Bennett race from “lean Republican” to “toss up” on Wednesday, citing Trump’s unpopularity in much of the district.

This is despite Comstock's months-long effort to jettison Trump. In April, the former lobbyist and state legislator said Trump was actually a Democrat who knows "nothing" about the economy.

She said in December that his plan to ban Muslim immigrants was "un-American" and "a silly idea." In March, she gave campaign donations she got from Trump to charity.

Earlier this month, she said Trump's boasts about groping women, revealed in a video tape, were "vile." She said she would not vote for him and urged Trump to drop out of the race against Democratic presidential nominee Hillary Clinton.

But it may be too late to undo the damage, political analysts said, with just 19 days remaining until Election Day.

"In a normal year, Comstock would be a clear favorite, but right now her front runner status is in question because of Trump," said Kyle Kondik, managing editor of Sabato's Crystal Ball at the Virginia Center for Politics.   

"He (Trump) was a gift" to her campaign, said Bennett, a 63-year-old real estate executive.

Bennett said Comstock, 57, waited too late in her re-election campaign to announce she would not vote for Trump, and argued that Comstock's views on immigration, abortion and climate change are "shockingly similar" to Trump's.

Asked on Wednesday about voters' view of her repudiation of Trump, Comstock said Bennett, if elected to the House, would be a "rubber stamp" for Clinton.

"I'm the only one of the two of us who ... has a record of speaking out against people, whether it's in my own party or not," Comstock, a Georgetown University-educated lawyer, told reporters after a debate with Bennett.

"I've made clear that I'm going to be my own woman."

Stretching from suburban Washington to the Shenandoah valley and the West Virginia border, Comstock's district is home to thousands of government workers, as well as many well-heeled lobbyists and what Trump might call the Washington "elites."

The area has been represented by a Republican in Congress since 1981, but has been a swing district in presidential years.

Bennett said Democrats expect about 140,000 more voters to turn out in the district in this presidential election year than when Comstock was elected in a mid-term election two years ago.

“We have a growing Latino community, we have a growing Asian community, and those communities are very unsettled by the rhetoric they’ve heard on the Republican side of the fence, and really worried about where this country is going,” she said.

Kondik said, "This is one of the most highly educated districts in the country, and it’s filled with the kinds of Republicans that Trump could very well turn off."

A recent poll by the Wason Center for Public Policy in Newport News, Virginia, showed Clinton leading Trump by 55 percent to 21 percent in northern Virginia.

That suggests that for Comstock to win, some Clinton backers will need to “split” their tickets to vote for Comstock, who made a name for herself in the 1990s as a congressional staffer dedicated to investigating members of the Clinton administration.

Stephen Farnsworth, professor of political science and international affairs at the University of Mary Washington in Fredericksburg, Virginia, said: "With the lewd tape, Trump went from being a drag on the Comstock campaign to an anchor."

(Reporting by Susan Cornwell; Editing by Kevin Drawbaugh and Alistair Bell)