O'Rourke campaigns in Houston, Thomson Reuters By Andy Sullivan

KATY, Texas (Reuters) - As Democrats try to win control of the U.S. Congress in next year's midterm elections, their hopes of picking up a Senate seat in Republican-dominated Texas rest with a telegenic ex-punk rocker who wants to impeach President Donald Trump and legalize marijuana.

Beto O'Rourke's long-shot bid to unseat incumbent Republican Senator Ted Cruz illustrates the tightrope Democrats must walk as they gear up for the November 2018 elections.

Democrats will have to win in areas that backed Trump last year in order to gain control of the House of Representatives and the Senate but even in Texas the party's left-wing base is pressing candidates to stop the president by any means necessary.

O'Rourke, who currently represents El Paso in the House, is drawing big crowds across Texas as he calls for universal healthcare and new restrictions on gun ownership. At a recent rally outside Houston, hundreds of supporters stood in line for up to an hour to shake his hand.

O'Rourke, 45, said Trump's surprise victory last year was a big reason he decided to run for the Senate. He said Trump's racially charged rhetoric and divisive governing style have led him to support impeachment.

"I'm now convinced beyond the shadow of a doubt that Donald Trump is unfit for that office," O'Rourke told Reuters in an interview.

"This is a moment unlike any other, certainly in my lifetime, I think in this country since the Civil War, where we have to really decide who we are, and there are two very clear paths that we can take," he said.

The end of the path laid out by Trump "is tyranny, it is not democracy," O'Rourke said.

UNAPOLOGETIC PROGRESSIVE

O'Rourke's unapologetic progressivism stands out among Democrats who are campaigning outside the party's liberal strongholds in the Northeast and on West Coast.

In deeply conservative Alabama, Democratic Senate candidate Doug Jones has been running as a pragmatist ahead of a Dec. 12 special election, saying he wants to be a "voice for reason" in Washington. He has run ads attacking Republican opponent Roy Moore over a sex scandal, but has steered clear of harsh anti-Trump rhetoric.

In Virginia, Ralph Northam denounced Trump as a "narcissistic maniac" as he sought the Democratic nomination for governor, but dialed back the rhetoric during the general election, telling voters ahead of his Nov. 7 victory that he would work with Trump when it was in his state's interest.

In Congress, a handful of rank-and-file House Democrats have filed articles of impeachment even though U.S. Special Counsel Robert Mueller is still investigating whether Trump's campaign worked with Russia in last year's election. Russia has repeatedly denied meddling and Trump has called the investigation a "witch hunt."

Democratic House Leader Nancy Pelosi said earlier this month that she would not make impeachment a priority if her party won the House next year but Democratic Senate Leader Chuck Schumer has said it is premature to consider impeachment.

Even some of O'Rourke's supporters say impeachment talk is counterproductive as long as Republicans control Congress. "Otherwise, in my view, it's just chest-beating," said Nikki Redpath, a Houston-area homemaker and O'Rourke campaign volunteer.

O'Rourke is seen as the favorite to win the Democratic nomination in March but analysts say his progressive views could prove a liability as he tries to reverse his party's long losing streak in the Lone Star State.

Trump finished nine percentage points ahead of Democrat Hillary Clinton in Texas last year and the state has not elected a Democratic governor or senator since 1994. Democrats have lost recent statewide elections by double-digit margins and have struggled to recruit top-tier candidates for major races.

Still, O'Rourke's anti-Trump message has resonated with oil-industry executive Katherine Stovring, who said she used to vote for candidates from both parties but now has been motivated to work for Democratic candidates as a way to stop Trump.

"I'm looking for ways to engage. This is our democracy at risk," she said.

Texas Republican strategist Matt Mackowiak said he thought O'Rourke would be trounced by Cruz unless voters turn en masse against Trump nationally.

"He's a more interesting candidate than the traditional sacrificial lamb the Democrats put up," Mackowiak said. "But he's far too liberal to be elected statewide."

In an era where differences between Republicans and Democrats are stark, candidates like O'Rourke have little incentive to moderate their positions, said James Henson, director of the Texas Politics Project at the University of Texas.

At this point there is little downside for O'Rourke to make polarizing statements on impeachment and other issues.

"I think you can expect to hear a lot more of that as the campaign unfolds," he said.

(Reporting by Andy Sullivan; Editing by Caren Bohan and Bill Trott)