Democrat Hillary Clinton opened up a substantial lead over Republican presidential nominee Donald Trump Thursday, according to a new national poll released Thursday.

According to the latest McClatchy/Marist poll, Clinton holds a 15-point lead over Trump, leading with 48 percent support to Trump's 33 percent, which marks the biggest lead for either candidate in any national poll since late March, when Clinton led by 18 points in a Bloomberg.

The poll shows a massive dip for the GOP nominee, who lost 6 percent support in just a month's time. In the previous McClatchy/Marist poll, the real estate mogul trailed his Democratic rival 42-39 percent ahead of each party's convention. On the flip side, it's an equally big jump that comes on the heels of her convention and a gaffe-filled week by Trump.

Over the past week, Trump has been mired in a public feud with the family of slain Army Capt. Humayun Khan after his father, Khizr Khan, bashed him in a speech at the Democratic National Convention.

Trump also continued to take aim at members of his own party. In a Tuesday interview with the Washington Post, Trump declined to endorse House Speaker Paul Ryan ahead of his primary contest next week, citing the support he's received from Paul Nehlen, his opponent.

The new poll isn't the only poor one on the day for Trump. According to Fox News' poll, Trump trails Clinton by 10 points, losing by a 49-39 deficit. New state polls in Pennsylvania, New Hampshire and Michigan all showed Trump to be trailing by significant margins.