(CNN) President Donald Trump is seizing on an attempt by 500 migrants to rush the southern border to use immigration as an issue to bolster his presidency at a critical political moment.

Trump spent weeks ahead of the midterm elections warning that the United States was about to experience an invasion from a migrant caravan trekking north across Mexico, and sent troops to the border in what critics branded a political stunt.

He seized on unrest at the frontier on Sunday to hike pressure on Mexico and to squeeze his political opponents in Washington as he demands financing for his border wall in a looming government funding showdown.

"Mexico should move the flag waving Migrants, many of whom are stone cold criminals, back to their countries," Trump tweeted early Monday morning.

"Do it by plane, do it by bus, do it anyway you want, but they are NOT coming into the U.S.A. We will close the Border permanently if need be. Congress, fund the WALL!"

Trump was reacting to reports that a group of migrants overwhelmed both Mexican federal and local police blockades and rushed towards the border at one of the busiest crossings between Tijuana in Mexico and San Diego. US authorities said several Border Patrol agents were hit by rocks, prompting the use of tear gas. Border crossings were closed in both directions for several hours, and it did not appear that any migrants breached the frontier.

The situation appeared to fall well short of the mass invasion by a caravan of thousands of migrants conjured up by Trump before the midterms.

Sunday's events will further electrify political tensions in Washington as it gets to work after the Thanksgiving holiday and ahead of a month of transition and increasing vulnerability for the President.

Trump is also set to travel to another big international summit this week -- the G20 in Argentina -- that carries the habitual possibility that he could again fall out with other world leaders. The trip will also bring him face-to-face with Chinese President Xi Jinping at the most contentious moment in US-China relations for decades as a trade war rages between the two powers.

Photos: Unrest at the US-Mexico border A member of Mexico's Federal Police tries to grab migrants as they attempt to cross the border into the United States on Sunday, November 25. Hide Caption 1 of 22 Photos: Unrest at the US-Mexico border A migrant family from Honduras runs from tear gas that was deployed by US Border Patrol agents near the fence between Mexico and the United States. Hide Caption 2 of 22 Photos: Unrest at the US-Mexico border Mexican police run as they try to keep migrants from getting past the border crossing in Tijuana. Hide Caption 3 of 22 Photos: Unrest at the US-Mexico border A group of migrants climbs the border fence between Mexico and the United States. Hide Caption 4 of 22 Photos: Unrest at the US-Mexico border Mexican police, right, stand guard as migrants walk on a bank of the nearly dry Tijuana River. The migrants were making their way toward the El Chaparral port of entry after circumventing a police blockade. Hide Caption 5 of 22 Photos: Unrest at the US-Mexico border Migrants and journalists run away from tear gas. Hide Caption 6 of 22 Photos: Unrest at the US-Mexico border Migrants return to Mexico after tear gas was deployed. Hide Caption 7 of 22 Photos: Unrest at the US-Mexico border A migrant covers his face after being affected by tear gas. Hide Caption 8 of 22 Photos: Unrest at the US-Mexico border Migrants are stopped by Mexican police officers before arriving at the port of entry. Hide Caption 9 of 22 Photos: Unrest at the US-Mexico border A migrant climbs the fence between Mexico and the United States. Hide Caption 10 of 22 Photos: Unrest at the US-Mexico border A migrant is pushed back as she attempts to break through a line of Mexican police to reach the border wall. Hide Caption 11 of 22 Photos: Unrest at the US-Mexico border A group of migrants crosses the riverbed of the Tijuana River in an attempt to get to the port of entry. Hide Caption 12 of 22 Photos: Unrest at the US-Mexico border Migrants hold up their hands as US Border Patrol officers detain them. They had jumped over the border fence between Mexico and the United States. Hide Caption 13 of 22 Photos: Unrest at the US-Mexico border US Border Patrol agents fire tear gas toward migrants at the border. Hide Caption 14 of 22 Photos: Unrest at the US-Mexico border Migrants cross the Tijuana River to reach the border fence. Hide Caption 15 of 22 Photos: Unrest at the US-Mexico border Migrants climb a metal barrier on the border. Hide Caption 16 of 22 Photos: Unrest at the US-Mexico border People attempting to cross into the United States gather near their vehicles after the San Ysidro Port of Entry was closed because of the migrant unrest. Hide Caption 17 of 22 Photos: Unrest at the US-Mexico border A migrant is stopped by US agents who order him to go back to the Mexican side of the border. Hide Caption 18 of 22 Photos: Unrest at the US-Mexico border Migrants are stopped by Mexican police officers. Hide Caption 19 of 22 Photos: Unrest at the US-Mexico border Migrants cross the riverbed of the Tijuana River as they try to get to the port of entry. Hide Caption 20 of 22 Photos: Unrest at the US-Mexico border Migrants clash with Mexican police. Hide Caption 21 of 22 Photos: Unrest at the US-Mexico border Migrants try to climb the border fence. Hide Caption 22 of 22

Democrats lining up investigations

Most ominously for Trump, Democrats are positioning themselves to unleash an unprecedented regime of investigation and oversight against his White House as soon as they take the reins of power in January, while special counsel Robert Mueller grinds away ever closer to the President's inner circle

Just how miserable things could get next year for the President was revealed by incoming House Intelligence Committee Chairman Adam Schiff on CNN's "State of the Union" on Sunday. The California Democrat vowed to bring acting Attorney General Matthew Whitaker before Congress to probe whether he was appointed with the express purpose of disrupting the Mueller investigation.

He said the President was not being honest in casting doubt on the CIA's conclusion that Saudi Crown Prince Mohammed bin Salman was responsible for the murder of Washington Post columnist Jamal Khashoggi.

Schiff also said the new Democratic House would investigate whether the President's response to this and other foreign policy questions was being influenced by a "hidden hand" of investments or past or business ties.

"Is his personal financial interest driving US policy in the Gulf? Is it driving US policy vis-a-vis the Russians? We don't know, but it would be irresponsible not to find out," Schiff said.

On NBC's "Meet the Press," Democratic Rep. Elijah Cummings of Maryland, who will head the House Oversight Committee, vowed proportionate investigations.

"The American people said to us through this election, 'We want accountability. We want to check on this President of the United States.' But they also said something else. They said, 'We want you to solve our problems,'" Cummings said.

A glimpse of how Washington will change

The prospects of flying subpoenas and administration officials being hauled up to Capitol Hill points to crucial dynamics ahead of the 2020 elections.

How will the President react? Will Democrats overreach and play into McConnell's warnings of "presidential harassment?" Can the President leverage Democratic opposition to court suburban voters who deserted him in the midterms? Or will any revelations either from congressional probes or Mueller prove to be so serious that they bury his White House in scandal and put his entire presidency at risk?

Trump could emerge from a period of retooling in his administration facing even fewer restraints from subordinates than has already been the case.

Key officials like Homeland Security Secretary Kirstjen Nielsen and White House chief of staff John Kelly are waiting to see if they will be retained. And Trump denied a Wall Street Journal report that he's getting irked by Treasury Secretary Steven Mnuchin over recent stock market losses.

Mnuchin is not currently thought to be threatened, but the report, and the President's frequent tweeting about oil prices and the markets, does appear to hint at some nervousness over the future of the economy, which in its currently roaring state is the best argument for his 2020 re-election hopes.

In a glimpse into the feudal nature of life in Trump's orbit, his former political aide Corey Lewandowski writes in his new book that enemies of the President are "embedded" in the White House and government agencies and trying to thwart his agenda, according to The Washington Post.

Trump has often embraced moments of converging threats and challenges to stoke an atmosphere of chaos in Washington that bolsters his standing with his political base and puts his opponents off balance.

But he now faces a test of his political strategy since his method -- for instance, using inflammatory rhetoric about immigration -- appears to have hurt Republicans in House races, even if it boosted Senate contenders.

For now, there is no sign of an adjustment: Trump broke convention in his Thanksgiving calls with troops by injecting political rhetoric about immigration and trade and boasting about his success.

His recent behavior, including a feud with Chief Justice John Roberts, either betrays an inability to rein in his relish for confrontation or a strategic decision that the wrecking ball approach is the best way to ensure his re-election.

Back on the border

Trump spent part of the weekend highlighting the migrant caravan.

"If for any reason it becomes necessary, we will CLOSE our Southern Border. There is no way that the United States will, after decades of abuse, put up with this costly and dangerous situation anymore!" Trump tweeted Saturday.

He may return to the topic on Monday in a pair of rallies for Mississippi Sen. Cindy Hyde-Smith, who is facing a runoff amid controversy over her past racially-tinged comments and actions.

The Washington Post reported that the incoming administration in Mexico has agreed to allow asylum seekers to remain south of the US border while their applications were processed, in what would be seen by Trump as a victory.

The new government, however, denied in a statement to CNN that a deal had been done.

A migrant is pushed back as she attempts to reach the border wall.

The politics of immigration will bear watching in the weeks ahead since they are crucial to Trump, who tends to resort to extremes, and Democrats who do not wish to come across as soft on border issues.

One Republican, Sen. Joni Ernst, who faces a potentially tricky Iowa re-election race in 2020, hinted at the delicate nature of an issue that electrifies Trump's base but can alienate more moderate voters on "State of the Union."

She said she would prefer that Trump not follow through on his threat to close the US border, though pointed to the reports from Mexico to argue his rhetoric could be working.

"I certainly think the President sees results any time that he does bring up an issue, and he does lay down certain reasons why he's doing what he's doing," Ernst told CNN's Dana Bash.

Ernst also said she hoped that Trump would not honor threats to partially shutter the government if he does not get his border wall funded.

Trump said last week that if there was ever going to be a shutdown over the border wall, now would probably be the time to do it.

Congress averted a shutdown in September by passing a massive spending bill to fund a large portion of the government. The package did not, however, include money for Trump's wall. Instead, lawmakers passed a shorter-term spending bill to fund the Department of Homeland Security, among other agencies, until December 7.

It's not clear either side really wants a shutdown. If precedent is any guide, it's also possible the funding crunch could be put off by short-term deals that run until the end of the year or beyond.

The calculations could also influence other priorities in the mix, including disaster funding requests for fire-ravaged California, and a drive by Democrats and some Republicans to shield Mueller.

Bipartisan anger over Trump's approach to Saudi Arabia could also complicate deal making, with both sides conscious of how actions now could play out in the new political reality next year.