The big work on growing the base involves securing the support of entire factions ~ not just individuals on the other side.
The three biggest modern transfers of factions involved black voters (North and South, and augmented through passage of special voting rights legislation and enforcement), Southern white voters, and Roman Catholic voters.
Today a full 40% of the effective voting strength of the Democrat party in states where they are dominant is made up of black voters. They control ~ but do not dominate the Democrat parties in those states, and in the cities where they exercise control they dominate politics with an incredible totality.
40% of the Republican base consists of Social Conservatives ~ and a great deal of that base arises out of social groups who did not regularly vote in the past, or simply didn't vote in elections in areas where traditional Republican Conservatives dominated politics. This is a 'new growth' situation, not like the 'voter transfer' situation encountered with black voters.
The old Republican merchant and professional class voters have been popularized as the new TEAParty ~ and they're about 40% of the Republican vote. The remaining 20% is divided up among a residual group in New England ~ true 'Bitter Enders' ~ and the GOP-e which is representative of disembodied financial interests and their hired professional politicians.
In all of my analyses of the situation ~ going back to the 1850s, I usually set aside Eisenhower and Grant as special cases. Neither one really needed to campaign. Both were considered representative of the victors in truly great wars ~ Civil War for Grant and WWII for IKE, and he had also been SUPREME ALLIED COMMANDER!
Even Truman wanted IKE to run for President as a Democrat.
It stands to reason that any successful candidate must secure and maximize his base and then motivate them to vote. It's easy to agree on that, it almost goes without saying.
My previous replies were to your original assertion that "usually you never get any votes from the other side". The opposite is the case with most winning candidates, and particularly those who have won by huge margins. It's just a matter of to what degree.
It would be logical to expect to never win any votes from the ideological core of the opposing party. That we can agree on.