WASHINGTON— Sen. Ted Cruz may face a competitive race in his bid for re-election in 2018, a poll released Wednesday shows.

The Texas Republican was tied at 30 percent with Rep. Beto O'Rourke, an El Paso Democrat who launched his campaign to challenge Cruz last month, in an early poll of the race conducted by the nonpartisan Texas Lyceum.

And Cruz trailed Rep. Joaquin Castro, a San Antonio Democrat expected to announce by the end of the month whether he will also challenge Cruz, by a slim margin of 35 percent to 31 percent.

Despite the unusually close numbers, a plurality of Texans — 37 percent — responded that they have not yet thought about the race, which is still 18 months away.

"Ballot tests conducted this far in advance of an actual election are, at best, useful in gauging the potential weaknesses of incumbents seeking re-election," said Daron Shaw, the poll's director.

Pointing to the high number of undecided respondents and Texas' consistent Republican support in recent decades, Shaw urged a "cautious interpretation" of the poll's significance. Conducted from April 3-9, the poll had a sample size of 1,000 Texans. Election questions were limited to 890 registered voters, with a margin of error of 3.3 percentage points.

Democrats performed strongly throughout the poll. Lt. Gov. Dan Patrick's little-known Democratic challenger Mike Collier narrowly topped the incumbent 27 percent to 25 percent -- within the 3.3 percentage-point margin of error. But even more Texas voters, 46 percent, said they had not thought about that race any more than the Senate race.

As President Donald Trump approaches his 100th day in office, his approval rating in Texas has gone underwater, according to the Lyceum poll. Forty-two percent of Texans approve of the Republican president -- who topped Democrat Hillary Clinton by 9 percentage points in Texas on election day — to 52 percent who disapprove of Trump.

The numbers excited Texas Democrats, who smell blood in the water ahead of the first midterm elections of Trump's presidency next year.

"It is entirely clear that Texans aren't buying what Republicans in Washington or Austin are selling," said Texas Democratic Party spokesman Manny Garcia. "Donald Trump's numbers are in the ditch, Texas Republicans are vulnerable, and our Democratic champions are poised for the battle of a lifetime in 2018."

But Republicans in the state expressed little concern about the figures, pointing to the long history of GOP success in deep-red Texas.

"Perhaps they've forgotten the results of statewide elections in Texas since 1994," said Texas GOP spokesman Michael Joyce of Democrats' enthusiasm around the early numbers, "but Texas won't be turning blue on Chairman Mechler's watch."

Chris Wilson, the former research director for Cruz's 2016 presidential campaign and the chief executive of conservative research firm WPA Intelligence, doused cold water on Democrats' hopes that Cruz is vulnerable, describing the poll as "fantasy-land."