OPINION — Only President Donald Trump could announce the parameters of his relationship with the newly Democratic House with a bite-sized limerick in his State of the Union address. “If there is going to be peace and legislation … there cannot be war and investigation,” Trump said with a did-you-see-what-I-did-there smile on his face. “It just doesn’t work that way.”

Was it a threat? Was it a poem? Is there a war happening that we don’t know about? Whatever Trump meant with his rhyme, it illustrated the very real challenge Democrats have on their hands, with a fleet of committee chairmen eager to investigate the president, a progressive base hungry for results, and a president who has never played by (and has often broken) the rules in his personal life, in his business affairs, and as president — right down to that little ditty in the State of the Union.

Unlike some past Congresses, which used to have to dig for years to find an area where they could effectively investigate the White House, Trump’s life and administration almost present more conflicts, curiosities and potential crimes than any Congress could digest. From the Russia conspiracy to the Trump family finances to the Trump hotel’s guest lists, it’s a target-rich environment. But the challenge for Democrats will be to not become part of the mess in the process.

For a White House that Democrats paint as out of control, the investigations into it have to be the opposite — namely serious, diligent and professional 100 percent of the time. Just one mistake or unhinged viral moment can give the president the tools he needs to delegitimize them all. Focus and restraint will be required in a moment, and in a capital, where both are in short supply.

Watch Trump blast former fixer Michael Cohen

The first test comes Wednesday, when former Trump fixer Michael Cohen will testify in front of the House Oversight and Reform Committee after lying under oath to Congress last year about the potential Trump Tower in Moscow. Cohen will likely be asked about the president’s potentially shady business practices; any possible violations of tax laws or campaign finance laws; and if I had to guess, those hush money payments to porn star Stormy Daniels.

Trump’s possible transgressions to this point in his life probably fall along a spectrum from offensive to inappropriate to completely illegal. But how much of it is worth Congress’, and thus the American people’s, time and attention? How much of it is relevant to the presidency? Democrats should have a strong sense of those answers before they go into that hearing, not after.

They should also know for themselves what their end game is. Is it impeachment of the president, even if that means 10 years of a President Mike Pence? Is it congressional oversight, law enforcement, or just a gigantic therapy session for progressives? Every question in open hearings — every line of inquiry, has to be tied back to those goals. But that’s a tall order for a dozen-plus committees and more than 200 members spinning off hundreds more ideas and hey-I-knows.

Just at the top of the list, in addition to anything that Robert Mueller’s investigation unearths, are:

— The House Judiciary Committee’s plans to probe the president’s emergency declaration to justify spending $7 billion on a wall at the southern border

— The House Veterans’ Affairs Committee’s review of a trio of “shadow rulers,” three friends of the president without military experience who met at Mar-a-Lago and may have improperly set policy for the VA

— A group of 10 House Dems who want to investigate Trump’s golf clubs in New York and New Jersey for hiring undocumented workers

— Another House Oversight investigation into former national security adviser Michael Flynn trying to sell nuclear technology to Middle Eastern countries

— Democrats on the Oversight, Foreign Relations, Intelligence, Financial Services and other committees demanding answers on the administration’s end to sanctions on three Russian firms tied to oligarch Oleg Deripaska

— Investigations into several current and former Cabinet secretaries’ outsize spending and travel budgets

— More committees looking into security clearance protocols at the White House

— A multi-committee look into possible Deutsche Bank connections to Trump, the Russian government and money laundering inside Trump Tower.

That’s just for starters. Are they all legitimate lines of inquiry? Probably. Is there any way the American people can follow and absorb all of them? Absolutely not.

So already, the Trump orbit is working to discredit all of the efforts en masse. On Monday, Donald Trump Jr. wrote every investigation in Washington off as a partisan effort to bring his family down.

“They’re not investigating actual crimes anymore,” Trump Jr. said on “Fox & Friends.” “They’re literally just trying to find something that they can make a big deal of.”

Separately, lawyers for the Trump Organization tried Monday to get House Judiciary Chairman Jerrold Nadler to drop his investigations, alleging a conflict of interest for a lawyer Nadler brought on from a firm that has worked for the Trump Organization in the past. Nadler and the firm both dismissed the accusations as a stall tactic. But you see where this is going.

So put this on the pile of challenges for Speaker Nancy Pelosi, who has shown an uncanny ability to keep most of her troops mostly in line. But she’ll need her sharpest whip to force her people to give up the little wins to get to the big ones. Last week, she told KQED radio that Democrats would be “strategic” in how they deploy subpoenas, “but we will not be delinquent.”

If Pelosi can keep her caucus between those guard rails, she’ll have managed the almost impossible and kept the integrity of her branch of government intact in the process. That’s more than we can say for Trump so far.