A few Democratic lawmakers -- who were riding in the Biden bus and hoping to stump for JB and Kamala Harris in Pflugerville, TX on Friday -- tweeted out the frightening encounter as it was unfolding ... claiming they had to abandon their plans for security reasons.

Those reasons, according to them, amounted to a small caravan of Trump supporters who'd been following them along their stops in Texas, and who allegedly harassed the bus when it made a stop near the Austin area. One local politician, Rafael Anchia, even claimed some of the Trumpers were armed, and appeared to be threatening the group inside if they stayed.

Some have claimed the President's own sons were somehow responsible for inciting the crowd to do this, but it's unclear where those accusations stem from. In any case, the trouble didn't stop after the bus decided to get outta Dodge ... because the same group that was protesting them on the ground reportedly hopped in their own pickups ... and gave chase!

There's a bunch of different videos that show at least three to four Trump-flag bearing vehicles tailing the Biden bus along I-35 for a good while, and getting awfully close. A Biden campaign spokesperson even reportedly said the convoy attempted to run the bus "off the road," and was creating a dangerous situation on the freeway for surrounding cars.

In one of the videos, you see a few trucks surround the bus and attempt to slow it down. It's pretty freaky -- but luckily, it doesn't sound like anyone was harmed ... and the Biden campaign was eventually able to get law enforcement to step in and escort them to safety.