Republican Sen. Ted Cruz looks to have a fight on his hands to keep his Senate seat in red Texas.

The conservative holds only a slim edge over insurgent Democratic Rep. Beto O'Rourke, according to an NBC News/Marist poll released Wednesday. Cruz garners 49 percent of support among registered voters in Texas, while 45 percent of respondents back O'Rourke. Six percent of registered voters are undecided.

The survey is the latest to show O'Rourke, a third-term representative, within striking distance of the first-term incumbent Cruz. An upset in Texas — which has not had a Democratic senator in about 25 years — would mark a major coup for Democrats, who face a brutal Senate map in this year's midterms.

O'Rourke has posted eye-popping fundraising totals and sparked grassroots enthusiasm in Texas. He has an opening in the state, where 46 percent of adults — a slight plurality — disapprove of the job Trump is doing as president, according to the NBC/Marist poll. Still, the state's Republican roots, and Cruz's own impressive cash haul, will make winning a challenge for the El Paso-area Democrat.

While O'Rourke has made the race closer, Cruz has generally led in public polling and still appears to be the favorite. Nonpartisan election analysis sites Cook Political Report and Sabato's Crystal Ball rate the race as "lean" Republican and "likely" Republican, respectively.

O'Rourke, a 45-year-old former punk rocker, has run a more liberal campaign than some would expect for a statewide candidate in a Texas. He favors abortion rights and has called for tighter gun restrictions. He has pushed for protections for young undocumented immigrants brought to the U.S. illegally as children.