You should see what Baltimore has done to exclude GOP voters
What, if anything, has Baltimore done to exclude Republican voters?
Bonus goalpost moving addendum: If the exclusion involves gerrymandering please show pictures!
You'll have to ask K Dog. The question was about city elections, so all these answers about states' gerrymandering of congressional districts are irrelevant. Here is what he was responding to:
"With the aim of explaining changes in electoral rules adopted by U.S. cities, particularly in the South, we show why majorities tend to adopt 'winner-take-all' city-wide rules (at-large elections) in response to an increase in the size of the minority when the minority they are facing is relatively small."
Note, as I said, it's taking about city election rules -- and not only that, but about at-large elections, which are the opposite of gerrymandering since they don't use districts at all. However, Baltimore's city council uses districts, with an at-large council president, so it's hard for me to guess what K Dog was referring to in response to this. See http://www.baltimorecitycouncil.com/council-members
The answers about gerrymandered congressional districts can't be right, because that is done by the state government, not by Baltimore... and in Maryland politics the DC suburbs are at least as powerful as Baltimore. See, for example, https://fivethirtyeight.com/features/growth-of-suburban-d-c-is-felt-politically-in-maryland/
Also, I hope I don't actually need to explain this on a politics site in a question about US politics, but Baltimore is not the capital of Maryland, so it wouldn't be used metonymically to refer to the state government; you would use "Annapolis" for that.
Gerrymandering is the big one. Maryland is two-thirds Democrat and one-third Republican, and in the 2010 redistricting, Democrats offset several large Republican-dominated rural areas by scooping up just enough of Baltimore to have a Democrat majority, creating absolutely crazy district lines. They then packed a district with all Republicans. This had the effect of making seven of Maryland's eight representatives come out Democrat and only one Republican, rather than the two or three representatives the Republicans might have had if the districts were more compact.
As you can see here, by at least one measure Maryland's gerrymandering caused them to elected one more democrat than would be expected with more fair districts. They currently have 1 Republican and 7 Democrats, so they would be expected to have 2 Republicans and 6 Democrats in the House instead.
538 did a good job of putting together an application that lets you see how gerrymandering could/does work: https://projects.fivethirtyeight.com/redistricting-maps/maryland/#Proportional
It depends on whether we're talking about the city of Baltimore, or Baltimore county. In terms of voters, the city of Baltimore is very heavily Democratic so any district that it's in is going to favor Democrats. However, voters in Baltimore county are much more evenly split overall, yet districts representing that county tend to elect Democrats.
(Since 'fair' districts are a topic that not even the Supreme Court is willing to decided on, for this answer I've defined 'fair' as being 'the same proportion of Congressional representatives as votes in presidential elections')
Baltimore city: According to the 2004, 2008, 2012, and 2016 presidential elections, Democrat/Republic voters in Baltimore city were split 82/17, 88/11, 87/11, 84/10, for a rough average of 85/12. There are three congressional districts that go through Baltimore city, the 2nd, 3rd, and 7th, and are all Democrat.
In order to be decently fair, 5 districts would be needed to give an 80/20 Democrat/Republican split. With only 3 districts, either Republicans have to be over-represented by 21 percentage points in a 66/33 split, or under-represented by 12 percentage points in the current 100/0 split. Whether it is more fair to heavily over-represent Republican voters in Baltimore city or have no (direct) Republican representation for them is a topic of a much broader discussion.
Baltimore county: Using the same election results above, Baltimore county had Democrat/Republican split of 52/47, 57/42, 56/40, 56/39, for an average of 55/42. There are four districts that go through Baltimore county, the 2nd, 3rd, and 7th as above, plus the 1st which is Republican.
With a voting split pretty close to 50/50, two Republican and two Democrat representatives would be decently fair. The last time it was even was in 2002, after which the 2nd district became Democrat. For the last 28 Congresses(i.e., since 1962), Baltimore county has had a Democrat/Republican split of 3/1 19 times, a 2/2 split 6 times, and a 4/0 split 3 times. For the Congresses since 2004, split has been 3/1 6 times and 4/0 once, for an average split that is definitely over-representing Democratic voters.