Can anyone help me understand what Democrats are doing with the debt ceiling?

Democratic leaders have proposed limiting the debt ceiling increase to three months, countering the Republicans’ proposal to increase it for eighteen months.  In a surprise move, Trump sided with the Democrats’ proposal and did so in front of Republicans at a joint White House meeting:

In embracing the three-month deal, Mr. Trump accepted a Democratic proposal that had been rejected just hours earlier by Speaker Paul D. Ryan of Wisconsin. Mr. Trump’s snap decision at a White House meeting caught Republican leaders off guard and reflected friction between the president and his party. After weeks of criticizing Republican leaders for failing to pass legislation, Mr. Trump signaled that he was willing to cross party lines to score some much-desired legislative victories.

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Congressional aides said privately that Republicans went into the meeting at the White House proposing an 18-month deal on government spending and the debt limit, only to run into resistance from the Democrats. They then proposed a six-month deal as a compromise, but Democrats insisted on a three-month agreement. Mr. Trump then surprised the Republicans by agreeing to the Democratic formulation.

Hmm . . . it is certainly an odd development on its face.  But what I can't figure out is why Democrats wanted only a temporary three month debt ceiling extension and blocked removing this dangerous issue for 18 months, as proposed by the Republicans?  

Normally, I would grab every additional month that this idiotic, existential threat is defused, and believe the debt ceiling concept should be repealed at the first possible opportunity. So Democrats should be pushing for a 2 year, or 3 year, debt ceiling increase, no?

Worse, reporting suggests that the Democrats’s approach is designed — like Republicans’ past misconduct — to turn the debt ceiling into a hostage/ransom scenario, for Democrats to attach unrelated policy goals:

Democrats say that this new deal with Trump strengthens their hand in passing the DREAM Act, which would provide a path to citizenship for these undocumented immigrants. They’ll be able to use the leverage they have on the debt ceiling to press for what they want legislatively, with the DREAM Act being high on the list.

Is this possibly true?  This would seem to be the worst, most destructive idea, with Democrats validating the dangerous Republican idea of recklessly toying with the debt ceiling for advantage by turning it into a bipartisan practice.

What am I missing here?  What, if any, are the benefits and the smarts of the Democrats’ approach here?