What are "uneven attempts"?

Solution 1:

And given Trump's uneven attempts to unite the nation in the past...

According to this link, "uneven" can be defined in the following way:

uneven: varying in quality (eg, an uneven performance)

Here, calling a performance "uneven" implies that it's inconsistent and varies in quality from time to time. Trump's attempts are described in a similar way. You could translate the sentence like this:

And given Trump's inconsistent attempts to unite the nation in the past...

Solution 2:

The definition you use is appropriate in the context of the quote, but it only points to inconsistency in Trump's attempts, rather than evaluating their merits or deficiencies. However, given Trump's record in this regard, I would view the use of "uneven" as pure irony. An ironic synonym for abysmal. Following your link, I see that the quote is from CNN, which makes abysmal a certainty.