What does the sinister journey mean in walt whitman's Ah, Captain?

"Oh, captain, my captain! Whitman wrote it in 1865 in memory of President Lincoln. The whole poem is ingenious in conception, clear in organization, profound in language and full of sincere feelings between the lines, expressing the poet's admiration and nostalgia for Lincoln. The sinister trip should mean that the president's action for the welfare of the people was opposed and obstructed by opposition forces.