Donald Trump is fuming after Vermont senator Bernie Sanders endorsed Hillary Clinton during a Tuesday event at a Portsmouth, N.H high school.

"There was a significant coming together between the two campaigns and we produced, by far, the most progressive platform in the history of the Democratic Party," Sanders said. "Our job now is to see that platform implemented by a Democratic Senate, a Democratic House and a Hillary Clinton president - and I am going to do everything I can to make that happen."

Although the democratic socialist politician was quick to point out that he doesn't agree with a number of issues in Clinton's campaign, the announcement came as no surprise, two weeks ahead of the Democratic National Convention and just before the Republicans formally nominate Trump next week in Cleveland.

"This campaign is about the needs of the American people and addressing the very serious crises that we face. And there is no doubt in my mind that, as we head into November, Hillary Clinton is far and away the best candidate to do that," Sanders said.

Trump voiced his displeasure with a short rant on social media.

The endorsement from Sanders comes a few days after the Green Party's Jill Stein offered to step aside, inviting him to take over her own party's ticket as its presidential candidate.

She said she reached out to him at the end of the primary season but had not received a response, the Guardian reported.

After news of Sanders decision emerged, Stein said many would be heartbroken, calling on Sanders supporters to vote Green.

But some environmental organizations that were rallying behind the senator applauded his decision. Friends of Earth Action president Erich Pica released a statement saying that Sanders' "inspiring" campaign allowed progressives and environmentalists to see the strength of speaking truth to power by offering bold solutions to the problems facing our country and planet."

Sanders, in fact, spoke about the risks posed by global warming during his endorsement speech. "This election is about climate change, the greatest environmental crisis facing our planet, and the need to leave this world in a way that is healthy and habitable for our kids and future generations. Hillary Clinton is listening to the scientists who tell us that if we do not act boldly in the very near future there will be more drought, more floods, more acidification of the oceans, more rising sea levels. She understands that we must work with countries around the world in transforming our energy system away from fossil fuels and into energy efficiency and sustainable energy – and that when we do that we can create a whole lot of good paying jobs. Donald Trump: Well, like most Republicans, he chooses to reject science – something no presidential candidate should do," he said.

Clinton's social media team reacted quickly to the senator's words by posting a Snapchat video.

Screenshot from Snapchat.

After Clinton defeated Sanders in primaries last month in California and five other states, and got endorsements by President Barack Obama and Vice-President Joe Biden, it was clear that Democrats were going to coalesce around her candidacy.

According to Vox's political writer Andrew Prokop, the reason why Bernie Sanders remained in the race until now is because he was trying "to maximize his leverage in shaping the Democratic Party’s platform," so it'd include key issues such as minimum wage and environmental policy. Prokop quotes Sanders adviser Warren Gunnells saying that his strategy paid off, since "at least 80 per cent" of the changes Sanders wanted ended up being included in the platform.

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