Honduran migrants heading in a caravan to the U.S., walk in Metapa on their way to Tapachula, Chiapas state, Mexico on October 22, 2018.

Not long ago, heart-rending pictures of immigrant children getting torn from their parents at the border spurred a bipartisan chorus of criticism. For a moment, it seemed that fear-based immigration tactics by the Republican Party might lose some force in the midterm elections.

More recently, however, images of thousands of Latin American immigrants walking toward the U.S. to seek asylum garnered a different reaction — especially from President Donald Trump, who warned without evidence that "unknown Middle Easterners" were among them.

The immigration issue has become a potent line of attack for Trump and the GOP just two weeks before the close of election season. The strategy: Link illegal immigration to crime, terrorism and economic competition and accuse Democrats of being responsible for it.

In a Monday morning tweet storm, the president railed against the Latin American migrants walking north toward the U.S. and blamed Democrats for blocking tougher immigration measures. He called Democrats "obstructionist" and implored House and Senate minority leaders Nancy Pelosi and Chuck Schumer, to "Call me!" He also threatened to cut off aid to Central American countries.

"Every time you see a Caravan, or people illegally coming, or attempting to come, into our Country illegally, think of and blame the Democrats for not giving us the votes to change our pathetic Immigration Laws!" he wrote in a tweet. "Remember the Midterms! So unfair to those who come in legally."

The caravan story has dominated conservative media outlets such as Fox News, where personalities have speculated that the caravan has links to terrorism. Other right-wing outlets have pushed conspiracy theories and baseless claims that the caravan includes Islamic terrorists, and that Democrats such as George Soros have supported and funded the effort. On Monday, an explosive device was found in the mailbox of Soros' suburban New York home.

Democrats have been wary of responding directly to Republican attacks, advised by strategists that a more productive route would be to focus on traditional Democratic issues like health care and taxation. But some strategists worry that cautious Democrats are not responding adequately to Republicans' foreboding immigration messages.

"If Democrats were any weaker, they'd be dishwater. The Democrat strategy has been, 'Let's watch Trump fall down on the issue.' The Democrats made their strategy a few weeks ago, focusing on health care. But a responsible party would be all over the immigration attacks, with intensity," said Hank Sheinkopf, a veteran Democratic strategist.

Pelosi and Schumer, in a joint statement on Saturday, said the president was "desperate to change the subject from health care to immigration because he knows that health care is the number one issue Americans care about."

An independent Grinnell College survey in late August and early September found that Republicans were more likely than Democrats and independents to be worried about illegal immigration. Fifty-two percent of Republican respondents incorrectly believed that illegal immigrants committed more violent crimes than the general population. Only 11 percent of Democrats and 27 percent of independents agreed.

Trump's strong rhetoric to stoke fears about foreigners mirrors his strategy in his 2016 presidential bid. Despite contrary advice from party pundits, he launched his campaign in 2015 by announcing that Mexico was sending "rapists" and drug dealers to the country. The statement garnered widespread criticism and outrage but ultimately helped him climb to the top of the Republican field of candidates.

It's unclear how the GOP's immigration rhetoric will play out on the entire midterm map, which encompasses more affluent suburbia and cities with diverse populations. But pictures of the caravan and right-wing media outlets' speculation will likely energize his political base, strategists said, which is predominantly made up of lower-income white voters who are already wary of immigration.