DENVER (AP) — The increasingly bitter U.S. Senate race in Colorado will test the viability of the Republican Party in a state that has shifted sharply to the left under President Donald Trump.

Republican Sen. Cory Gardner has widely been considered the most vulnerable GOP senator because of that change in his native state. With the exception of a single board of regents member, Colorado Republicans have lost control of the state Senate and every statewide race since Trump was their presidential nominee in 2016.

Gardner, elected narrowly in 2014's wave year for the GOP, rescinded his support of Trump in the final days of the 2016 election, after the release of the Access Hollywood tape. But he enthusiastically endorsed Trump's reelection and his relationship with the president has become a central issue for his Democratic challenger, former Gov. John Hickenlooper.

Hickenlooper and Democrats have repeatedly tied Gardner to the president, noting that Trump told a packed Colorado Springs rally in February that Gardner “has been with us 100%.”

Gardner, 46, has had to walk a fine line, touting his own bipartisan achievements of a sweeping public lands bill and a national suicide prevention hotline without being critical of the president and alienating Trump's base, which he needs to win re-election.

The problem for Gardner and Colorado Republicans is that Trump's base won't be enough to win. Democrats have overtaken Republicans in party registration since 2014, while unaffiliated voters — the largest share of voters in the state — have turned sharply against the president.

Gardner's vulnerability initially drew more than a dozen Colorado Democrats to seek their party's nomination to face him this year. Hickenlooper, perhaps the most prominent Democrat in the state, initially passed, instead mounting a brief, ill-fated bid for the Democratic presidential nomination. But national Democrats convinced him to instead challenge Gardner.

The former governor had previously bemoaned negative partisan campaigning. But his race against Gardner swiftly turned nasty. The incumbent senator and his GOP allies highlighted how Hickenlooper was found to have twice violated state ethics law by the independent ethics commission. Hickenlooper, 68, improperly took a private flight and a limousine ride in Italy while governor, the commission found.

Hickenlooper dismissed the matter as a “technical" error. Gardner called Hickenlooper “the most corrupt governor in Colorado history.” Hickenlooper, admitting he was being negative this time, dismissed Gardner as a silver-tongued politician. Last week he released a video of himself reading Colorado-themed insults against Gardner that people had posted on Instagram.

The two men had worked well together and respected each other during Hickenlooper's two terms as governor. But their increasingly barbed race was a reflection of the polarized turn in both national and state politics in recent years.

With Republicans struggling to hold Senate seats in conservative states such as Kansas and South Carolina, Gardner is considered the underdog in Colorado. Hickenlooper came to prominence in Colorado nearly 20 years ago when he became Denver mayor. His victory in the 2010 gubernatorial race helped establish Colorado as a politically competitive, slightly Democratic-leaning state. Democrats hope that he will now dispatch the most prominent remaining elected Republican.

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