While Donald Trump's favorability rating continues to decline so has the percentage of Republican voters who believe he will become their party's presidential nominee, a new poll shows.

A Rasmussen Reports survey taken in mid-March, shortly after Trump won a handful of primaries including Florida's winner-take-all contest, found that 87 percent of GOP voters believed the New York billionaire was "likely" to be the Republican nominee.

But the latest survey released Friday shows that in three weeks time, the percentage of Republicans convinced a Trump nomination will happen has declined 10 percentage points. While 46 percent of all voters in mid-March said Trump was "very likely" to be the GOP nominee, only 28 percent feel the same way now.

The steady decrease of GOP voters' confidence in Trump comes as the billionaire struggles to regain momentum following a tumultuous two-and-a-half weeks for his campaign.

Trump's landslide loss in Wisconsin on Tuesday has further fueled speculations that he will not reach the 1,237 delegates needed to secure the GOP nomination the convention in July, and his rapidly declining favorability ratings have exacerbated Republicans' concerns about his electability in the general election.

Rasmussen's latest survey of 1,000 likely U.S. voters was conducted April 5-6. Results contain a margin of error plus or minus 3 percent.