While I realize that Donald Trump did not necessarily have a wide choice of high-level politicians willing to become his running mate and therefore Mike Pence may have been the least bad of all available options, his selection still strikes me as a bad move.
Trump has acknowledged that his goal was party unity. However in order to achieve it, he needs to appeal to both Cruz voters and Kasich voters. The Pence pick is clearly aimed at winning over Cruz voters.
But it is actually unclear how effective it can be -- in a sense of gaining actual votes for Trump. Very conservative voters tend not to care very much about personalities -- what they want is ideological purity. Some of them are already attacking Pence for "selling out" to Trump (as well as for his past deviations). And they won't be happy with Trump unless he adopts all their positions. Ironically, most of Cruz voters would vote for Trump anyway (even without Pence on the ticket) because of their hatred for Hillary Clinton. And even if they don't, the worst (for Trump) they will do is stay home, write somebody in or vote for Gary Johnson -- but they won't vote for a Clinton under any circumstances.
That is not the case though with Kasich voters -- quite a few of them may end up voting for Hillary. They don't care about Trump's ideological heresies -- they worry about what he might do to their jobs and 401(k)s. They often have the highest incomes and education levels among Republicans, and they typically suffered the least or even came out ahead in the great socioeconomic dislocations of the past couple of decades (due to both globalization and Information Revolution). They dislike (but not hate) Hillary and view her as dishonest and corrupt -- but they also view her as safe. At the same time they tend to agree with the Clinton campaign portrayal of Trump as too risky. They are noticing that on multiple occasions Trump recklessly suggested he might default on the national debt, that he talks about huge tariffs and renegotiating long-standing trade deals, that his foreign policy ideas, while not very likely to lead to a major war, are quite likely to increase global instability and damage the economy...
It is precisely because of such concerns that the last Republican Treasury secretary endorsed Hillary Clinton. He wrote, "I can't help but wonder what would have happened if a divisive character such as Trump were president during the 2008 financial crisis, at a time when leadership, compromise and careful analysis were critical. The only reason we avoided another Great Depression was because Republicans and Democrats joined together to vote for the Troubled Asset Relief Program -- a vote that they knew would be politically unpopular but in the best interest of our country." Until last week Trump actually had a partial defense against that argument. He could point out that unlike most House Republicans, he publicly supported TARP at the time (as well as the auto-bailout and the economic stimulus), and he was even attacked for that in the primaries. But now Trump is defenseless, because the powerful House opposition to TARP was led by none other than Mike Pence (I wrote about his role and presidential aspirations five years ago). Granted, TARP remains unpopular among Republicans. But a lot of Kasich voters have a different view, basically agreeing with Hank Paulson's assessment that TARP was "[t]he only reason we avoided another Great Depression". So many of them will be horrified when they learn how Pence almost stopped it (and later was an enthusiastic supporter of other reckless economic ideas from playing chicken with the debt ceiling to budget sequestration). Of course, they will also be mindful of the fact that Pence has a real chance of actually becoming president -- not only because Trump would be the oldest first term president ever, but also because he might well become bored or frustrated with the presidency and resign.
So if anything, Trump's selection of Pence will only reinforce perceptions of him as too risky among some important segments of the Republican Party as well as among many independents. This will not merely suppress Trump vote but will actually increase Clinton vote -- and also make the upper middle class and Wall Street donate less to Trump and more to Hillary.