24 Democrats are running for president, all looking to unseat President Donald Trump in 2020. But still hammering Hillary Clinton, who is not running.

Trump made Clinton a major theme of his reelection campaign launch earlier this week.

For Trump, Clinton is representative of the many ongoing probes into his administration and personal finances, which can cause considerable damage to his presidency.

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At the moment there are 24 Democrats running for president, all looking to unseat President Donald Trump in 2020. That means the incumbent Republican has to wait quite a while to have a clear challenger on the Democratic ticket in his reelection campaign.

In the meantime, Trump is repeatedly hammering his 2016 foe, former Secretary of State Hillary Clinton, every chance he gets, making her his primary target.

Read more: No sitting president has survived a serious primary challenge in the past 50 years. Here's why Trump should be worried.

"We went through the greatest witch hunt in political history," Trump told a packed arena of supporters at his official campaign launch in Orlando, Florida earlier in the week. "The only collusion was committed by the Democrats, the fake news media, and their operatives, and the people who funded the phony dossier: Crooked Hillary Clinton and the DNC."

Trump tied Clinton into the special counsel investigation that dogged his administration for nearly two years, which he regularly characterizes as a "witch hunt."

"They tried to take away your dignity and your destiny. But we will never let them do that, will we?" he said. "They tried to erase your vote, erase your legacy of the greatest campaign — probably the greatest election in the history of our country ... They want to destroy you."

Trump also sees Clinton's face quite a bit due to how frequently Fox News — the president's favorite cable television network — mentions her on air.

Even years after the 2016 election, Fox covers Clinton and puts her on screen with much greater frequency that competing cable news channels, according to data from the TV News Archive.

For Trump, Hillary Clinton represents the many people in Washington who want to undermine his presidency

It makes sense as to why Trump stays so focused on Clinton. To him, she is representative of what he has to face day in and day out: A Washington establishment hellbent on undermining his presidency and not letting him do the things the way he wants to do them.

Since taking control of the House of Representatives in the 2018 midterm elections, Democrats have ramped up investigations into every aspect of the Trump administration. Combined with the special counsel probe, Trump has more opponents in Washington than he can count.

Read more: Here are all the insulting nicknames Trump has used for 2020 Democratic candidates

And Trump's supporters love it. Throughout Trump's rallies, of which he has held many since the 2016 election, chants of "lock her up!" break out all the time.

Trump regularly invokes Clinton's name at his rallies, questioning why the FBI did not pursue investigations into her while lamenting the ongoing congressional probes into his associates and administration.

One former Clinton campaign aide told INSIDER that the GOP base is conditioned to still hate Clinton, so it makes sense that Republicans and Trump in particular would want to keep focusing on her. Essentially, the aide said, is that "you find a thing that works and you keep pounding it until it no longer works."

"[Trump's] campaign is basically one giant engagement campaign focused on doing stuff to get his base fired up," the aide said. "That's all part of the feedback loop."