Jeb Bush's standing among Republican primary voters continues to improve, Marco Rubio remains popular with the party and two-thirds of GOP voters say they couldn't support Donald Trump.

Those are the findings from a brand-new NBC News/Wall Street Journal, which was conducted June 14-18 — during the build-up and coverage of Bush's and Trump's official presidential announcements.

According to the poll, 75 percent of Republicans say they could see themselves supporting Bush, the former Florida governor, for president, versus 22 percent who couldn't (+53 points). That's up from his 70 percent-27 percent score in April (+43), and 49 percent-42 percent score in March (+7).

Rubio, the Florida senator, has the highest margin of support among Republicans, with 74 percent who could see themselves backing him and 15 percent who couldn't (+59) - unchanged from April's poll.

Meanwhile, Wisconsin Gov. Scott Walker, who is expected to announce his presidential bid next month, has plenty of upside with GOP voters: 57 percent could see themselves supporting him, versus 19 percent who couldn't (+38), but there's another 21 percent who say they don't know his name.

The most unpopular GOP presidential candidate in the poll? Donald Trump, who announced his White House bid last Tuesday (but who still has yet to file his "statement of candidacy" with the Federal Election Commission). Just 32 percent of Republican primary voters say they could see themselves supporting him, compared with a whopping 66 percent who couldn't (-34).

Below are the scores on this question for the full GOP field: