3:53 PM PT -- Tim Walz is using his first speech on the campaign trail to take shots at Donald Trump, and the crowd is eating it all up.

Walz ripped Trump's presidency, saying DT froze in the face of the COVID crisis and tanked the economy ... then he delivered the zinger.

"Make no mistake, violent crime was up under Donald Trump ... and that's not even counting the crimes he committed."

3:43 PM PT -- Huge reaction from the crowd when Kamala Harris and Tim sent a message to Donald Trump and others who they say want to "turn back the clock on our fundamental freedoms."

Kamala declared ... "We're not going back!!!" Then the crowd started chanting it along with her and Walz.

3:10 PM PT -- Kamala Harris and Tim Walz hit the stage and the crowd went wild as the current VP says she solidified the Democratic nomination for the 2024 presidential election.

Kamala Harris and Tim Walz are hitting the ground running ... holding their first campaign rally together after Democrats finalized the top of their ticket.

The Vice President and the Minnesota Governor are in Philadelphia speaking to supporters Tuesday ... many of whom lined up in the rain just to get into the event.

Harris announced Walz as her running mate this morning, and the Philly rally is key because Pennsylvania is a huge swing state in this year's election ... with Harris and Donald Trump currently tied in the polls.

In selecting Walz, Harris passed over Pennsylvania Governor Josh Shapiro ... who is super popular in the battleground state ... but it looks like there's still a lot of enthusiasm for Harris here, even without Shapiro.

Harris and Walz are doing a campaign blitz this week ... after Philly, they're going to Wisconsin, Michigan, Arizona and Nevada ... making one stop per day in states where they need to win Electoral College votes.